Dissuade, frighten off (5)
I believe the answer is:
deter
I believe this is a double definition.
'dissuade' is the first definition.
(I know that dissuade can be written as deter)
'frighten off' is the second definition.
(I've seen this in another clue)
(Other definitions for deter that I've seen before include "Warn off" , "Prevent by instilling fear" , "'Discourage, dissuade (5)'" , "Try to prevent" , "Discourage (someone from an action)" .)